The present invention relates to catalysts endowed with high catalytic stability for the production of chloroform (CHCl3) starting from carbon tetrachloride (CCl4) by catalytic hydrodechlorination (HDC), to the method for producing the catalysts, and to the process in which the catalysts are used.
CCl4, together with chlorofluorohydrocarbons, has recently been banned (production and use) due to the intense depletion of the stratospheric layer of ozone due to this compound.
However, CCl4 is formed as a main by product in the reaction for the synthesis of chloromethanes from methane and chlorine.
The disposal of CCL4, typically performed by incineration, in addition to being onerous, is a considerable environmental problem.
An advantageous alternative to disposal by pyrolysis is catalytic hydrodechlorination to chloroform.
CHCl3 is a useful product which does not have the environmental problems of CCl4 and is therefore not subject to production and/or utilization constraints: it is in fact used in the production of the new fluorohydrocarbons, which do not interfere with the stratospheric ozone layer.
It is difficult to realize an industrial process which allows to limit the generation of unwanted chlorinated byproducts produced by the variety of reactions involved in the HDC process and to work with catalysts which maintain their activity substantially unchanged for a sufficiently long time, due to the fact that the above cited reactions are highly exothermic and because of the ease with which the catalyst can be poisoned by the hydrogen chloride generated during the reactions and/or by contamination by carbon-containing byproducts.
A sufficiently high selectivity can be achieved by working with low conversions and high hydrogen/CCl4 ratios; however, these conditions are of no practical interest.
Patent literature reports various methods for maintaining a high stability of the catalyst and reducing the generation of byproducts.
U.S. Pat. No. 5,105,032 describes catalysts based on Pt supported on oxides having a large surface area, in which the catalysts are pretreated at the temperature of 200xc2x0 C. or higher with hydrogen chloride (before or during reduction with hydrogen) in order to give the catalyst high resistance to deactivation.
The hydrodechlorination reaction is preferably performed in the presence of HCl, using CCl4:HCl ratios between 1:1 and 1:2.
An excess of HCl has the effect of lowering conversion but increases selectivity. Platinum is preferably used in a mixture with promoters such as Sn, Sb, Bi, Ti, Pb and mixtures thereof.
The above cited patent describes that a commercial catalyst containing Pt supported on gamma alumina and not previously treated with HCl loses its activity already after a few hours of reaction.
Because of pretreatment with HCl at high temperatures and of the need to proceed by reaction in the presence of significant concentrations of HCl, the above cited process is of limited practical interest.
EP-A-570020 describes a catalyst for HDC which comprises Pt supported on alumina having a surface area of less than 100 m2/g, mainly constituted by alpha, theta or delta alumina or mixtures thereof according to the calcination temperature.
Applied Catalysis A. General 174 (1998) 33-39 describes a catalyst containing Pt supported on gamma alumina which has high resistance to aging and is obtained by treating with an aqueous solution of NH4Cl at ambient temperature a commercial catalyst based on Pt supported on gamma alumina.
Treatment with NH4Cl has the effect of increasing the size of the Pt particles, which are mostly smaller than 0.5 nm and are arranged in an egg-shell configuration on the supporting particle.
In the treated catalyst, the particles have a size of 5 to 8 nm.
The high reactivity with HCl and CCl of the crown of relatively small particles of Pt, which are also in a state of considerable electron deficit, is thought to be responsible for the poisoning of the catalyst on the part of HCl.
In the untreated and deactivated catalyst, most of the Pt is in the form of chloride, Ptn Clx.
The treated catalyst maintains a substantially constant conversion and selectivity (between 70 and 80%) for over 2000 hours.
The aim of the present invention is to provide a catalyst for hydrodechlorination of CCl4 to CHCl3 containing Pt supported on gamma alumina which maintains its high activity and selectivity for CHCl3 substantially unchanged for sufficiently long periods without having to subject the catalyst to activation pretreatments.
This aim and other objects are achieved with the catalyst according to the invention.
It has been found unexpectedly that it is possible to obtain catalysts based on Pt supported on gamma alumina which have high activity and selectivity in the reaction for hydrodechlorination of CCl4 to CHCl3 and are able to maintain their catalytic performance practically unchanged for very long periods (2000 hours and more) if the catalyst is prepared in conditions which allow the platinum to be dispersed throughout the mass of the supporting particle instead of only on the surface layer, as occurs in conventional catalysts containing platinum supported on gamma alumina.